1. Field of the Invention
The present invention generally relates to the fabrication of stencil or membrane masks for high-resolution lithography, and more particularly to an improved method of forming a substrate for such masks.
2. Description of the Related Art
During the fabrication of stencil or membrane masks for high-resolution lithography, one or many membrane regions 10 are defined in a thick substrate material, as shown in FIG. 1. Small spaces or struts 11 between these membranes 10 (the membranes often referred to as membrane subfields) act as mechanical support for the thin membranes 10. It is advantageous for these struts 11 to be vertical to maximize the useable area on the mask surface. The membranes 10 are typically 1 mmxc3x9712 mm for e-beam projection lithography (EPL) and may be as large as 50 mmxc3x9750 mm for x-ray (including Extreme Ultraviolet (EUV)) or ion projection lithography (IPL). The effective field sizes are defined by the exposure system and the tolerances on the membrane dimensions are large.
The membrane regions 10 are typically defined by patterning a resist layer on the backside of the substrate (almost always silicon) and then etching the silicon through a wet etch or Reactive Ion Etch (RIE) using a hardmask. The wet etch either leaves tapered profiles (from the crystalline silicon (100) planes) which reduce the useable area of the mask, or requires silicon (110) wafers that etch along the crystal planes to leave vertical strut profiles, but are more expensive and produce vertical profiles in only one direction. The wet etch is designed to stop on the membrane material with high selectivity. Typically, the wet etch takes several hours (but can be done in batches).
For example, U.S. Pat. No. 5,464,711 to C. J. Mogab et al. (hereinafter xe2x80x9cMogabxe2x80x9d), discloses that defining the membrane regions with an RIE allows vertical strut profiles in all substrate materials with all orientations. However, the process in Mogab takes several hours per substrate (and usually is a single wafer process) which is expensive in a dedicated RIE system. Further, with the system in Mogab, the RIE must also stop on the membrane material and since RIE""s are typically less selective than wet etches, the etch is either stopped close to the membrane (and finished with a wet etch) or some barrier layer is used underneath the membrane material.
Other convention methods of forming masks include electrodeposition of metal onto a substrate and subsequent removal of the substrate. This, however, limits the choice of substrate and membrane materials and is rarely used in practice.
Therefore, there is a need for a method and system that forms membrane masks utilizing a simpler and less expensive processes. The invention described below provides a process which reduces the cost and time required to form such membrane masks.
The following disclosure describes a process for forming subfield regions by mechanical definition of the substrate through machining or mold forming. The subfield regions are then filled with a sacrificial layer before the thin membranes are deposited.
More specifically, the invention mechanically machines slots through a substrate (the slots have dimensions of membrane subfields), fills the slots with a sacrificial material, planarizes the substrate, deposits a membrane material over the substrate, patterns the membrane material and removes the sacrificial material. In a different but related embodiment, the invention utilizes a mold to form the slotted substrate in place of the machining operation.
In another embodiment, the invention deposits a lithographic mask material on a sacrificial material, machines slots in the substrate, attaches the sacrificial material to the substrate, removes the sacrificial material, and patterns the lithographic mask material.
The machining involves applying a cutting tool to the substrate to form the slots. Before the depositing of the membrane material, the invention can optionally deposit an intermediate layer, to assists the nucleation of the membrane material. The patterned membrane material regions are positioned above the slots. The sacrificial material provides support for the lithographic mask material.